Abstract American youth spend more time with media than in any other activity. Almost a third of this time is spent simultaneously engaging with (or switching between) multiple media streams (`media multitasking'; MMT). The rapid rise in MMT has generated considerable scientific and societal interest in determining whether, and if so how, MMT impacts cognition, psychosocial health, academic achievement, and brain structure and function. Given the prevalence of MMT in children and young adults, there is urgency to understand the neurocognitive profiles of media multitaskers. Mounting evidence (including new behavioral and neural findings generated in our exploratory grant R21MH099812) points to consistent MMT-related differences in attention and memory, yet the nature and extent of these cognitive differences, their neural underpinnings, and their emergence across the lifespan remain largely unknown. The proposed multi-modal research program will examine the neurocognitive differences associated with chronic MMT, and will explore how and when differences onset. In young adults (18-24yr), we will leverage sensitive behavioral paradigms, scalp EEG, and concurrent EEG-fMRI to test hypotheses about the nature and neural underpinnings of MMT-related attention and memory differences, and we will relate these cognitive and task-based neural differences to functional and structural connectivity in frontoparietal networks of attention and cognitive control. In children (7-12yr), we will use a large-sample, longitudinal design and a novel multi-domain cognitive assessment tool to measure working memory (WM), selective and sustained attention, and inhibitory control; we will relate these cognitive measures to MMT behavior and to EEG measures of attention- and memory-related neural function. We also will obtain measures of academic achievement for both age groups. Our aims are to: (Aim 1) delineate the nature of the WM and long-term memory (LTM) decrements associated with heavier MMT, and (Aim 2) test the hypothesis that increased attentional lapses and diminished attentional control contribute to these decrements; (Aim 3) test the hypothesis that the large-scale frontoparietal structural and functional networks that support attention and cognitive control vary with MMT; and (Aim 4) determine how and when MMT-related cognitive and neural differences arise in children (7-12yr), at the time when younger populations begin to engage in MMT. The proposed research will (a) delineate how memory, attention, and cognitive control, and their underlying neural systems, vary with MMT and relate to academic achievement in young adults and children, as well as (b) begin to shed light on whether chronic MMT is a cause or consequence of neurocognitive differences. Moreover, by leveraging an individual differences approach, we will advance mechanistic understanding of the interactions between four R-DoC cognitive constructs (attention, cognitive control, WM, and declarative memory) and their dependence on large-scale frontoparietal structural and functional networks.